A recently developed printing process involves coating of an image-receiving sheet with a layer of subminiature capsules. Each capsule contains a radiation sensitive dye, or ink, composition. It may be noted that the dye sometimes requires a developer in order to color the image-receiving sheet, and such a dye precursor or dye is herein referred to simply as a dye. A pattern of light of the sensitizing wavelength is directed at the layer of capsules, such as light reflected from an original to be copied or directed through multiple miniature shutters controlled by data signals. The light increases viscosity, or tends to harden, the liquid contents, sometimes herein referred to as dye, in those capsules on which it falls. When the layer of capsules are against the image-receiving sheet, pressure is applied to the capsules to rupture them and flow dye from those capsules not exposed to light, onto the image-receiving sheet and print an image thereon. Those capsules whose ink composition has been hardened can still be ruptured, although they resist rupturing more than non-hardened capsules. Furthermore, even when a hardened capsule is ruptured, its viscous dye will resist flowing and it generally does not release dye.
Pressure for rupturing the capsules has often been accomplished by placing the image-receiving sheet with a layer of capsules thereon between a pair of pressure rolls. The pressure rolls would exert forces of 200-400 pounds per linear inch to break the capsules. Such pressure rolls have the disadvantage that they can calendar the paper, and the pressure rolls have to be large and precision made and mounted, with the difficulty increasing as the width of the image increases. Such pressure rolls which apply high pressures have a tendency to wrinkle the paper and to cause paper jams. Another approach, shown in U.S. Pat. No. 4,448,516 involves the application of a brush which directly brushes the layer of capsules to rupture them and allow the dye to flow into the sheet which carries the capsules. This has a disadvantage that some of the dye can be picked up by the bristles of the brush and applied to areas of the image-receiving sheet which are not to be dyed (at least with that color of dye). Printing systems which could use photosensitive rupturable capsules, and which avoided the disadvantages of prior rupturing systems, would be of considerable value.